1. Field of the Invention
The present invention relates to a diffraction grating and an optical pickup used in an optical information processor that can record and reproduce information into and from a plurality of recording media with different standards, such as CD (compact disk), DVD (digital video disk/digital versatile disk), or Blu-Ray disc, using different wavelengths.
The present invention also relates to a diffraction optical element and a fabrication method thereof, as well as to an optical pickup device using the diffraction optical element and an optical disk drive that includes the optical pickup device.
2. Related Art
In an information processor for recording and/or reproducing information into and from a recording medium, a light beam is conventionally used to record and reproduce the information into and from the recording medium. A typical example of optical recording and reproducing is a system using a DVD-standard disk as the optical recording medium, which is commercially available. This system was produced in response to a demand for recording video data with contents of two hours or more and compressed/coded based on MPEG 2 in one side of an optical recording medium with a diameter of 12 cm. Under the DVD standard, the storage capacity of one side of the disk is 4.7 GB, the track density is 0.74 μm/track, and the linear density is 0.267 μm/bit. Hereinafter, a disk based on the above-described DVD standard is referred to simply as a “DVD”.
Information recorded on an optical recording medium, such as a DVD, is reproduced using an optical head. With the optical head, the light beam emitted from an LD (or a semiconductor laser) is focused onto the pit line formed on the track of the optical recording medium by an object lens. The light beam reflected from the optical recording medium is guided onto a photodetector by a condensing lens, in which a reproduction signal is generated. The reproduction signal produced by the photodetector is input to a reproduction signal processor to decode the data. In the case of DVD, the wavelength of the LD of the optical head is 650 nm, and the numerical aperture (NA) of the object lens is 0.6.
Blu-Ray Disc is a new standard that improves the density of the DVD standard, using a blue-violet laser with a wavelength of 405 nm. The Blu-Ray Disc standard was proposed for the next-generation large-capacity optical recording medium, which allows a maximum of 27 GB of video data to be repeatedly recorded and reproduced into and from a single layer of one side of a phase-change optical recording medium with a diameter of 12 cm.
For Blu-Ray Disc, a shortwave blue-violet laser is used, and the numerical aperture (NA) of the object lens for condensing the light beam is set to 0.85 in order to reduce the size of the beam spot. The Blu-Ray Disc recording medium employs a transparent protecting layer with thickness of 0.1 mm, in accordance with the increased numerical aperture of the lens. This arrangement reduces the aberration due to inclination of the optical recording medium and reading errors, while improving the recording density. Consequently, the recording track pitch of the optical recording medium can be reduced to 0.32 μm, which is about half of that of the DVD, and the maximum 27 GB of high-density recording is realized on one side of the Blu-Ray optical recording medium.
FIG. 1 is a schematic diagram illustrating a conventional pickup device for recording and reproducing information onto and from an optical recording medium (a DVD). An optical pickup 101 generally employs a polarizing optical system. A PBS (polarizing beam splitter) 104 is placed on the optical path extending from LD (or the light source) 102 to the object lens 106. The light component having the same polarization plane as the linear polarization of the LD 102 passes through the PBS 104, and is rendered to circularly polarized light by a quarter-wave plate 105. The circularly polarized light is focused through the object lens 106 onto the recording layer of the optical recording medium 108, which is formed beneath top surface of the substrate.
The light reflected from the reflecting surface of the optical recording medium 108 is also circularly polarized light, but rotated in the opposite direction to the incident light. The opposite-circularly polarized light passes through the quarter-wave plate 105 and becomes linearly polarized light with a polarization plane perpendicular to the polarization plane of the LD 102. The linearly polarized light is reflected from the PBS 104 and guided to the PD (photodetector) 110 via the condenser 107. When the quarter-wave plate 105 produces perfectly circularly polarized light, the component of the return beam having passed through the PBS 104 becomes zero, and the entire light beam reflected from the optical recording medium 108 is detected by the PD 110.
Meanwhile, a variety of optical pickups, which are used in an optical disk drive (i.e., an optical information processor) and have an optical system including a diffraction grating, have been proposed. In this type of optical pickup, the light beam reflected from the optical recording medium is diffracted by the diffraction grating, and detected by the photodetector. One type of known diffraction grating is a polarizing diffraction grating. For example, JPA 9-50642 discloses an optical head and a fabricating method thereof. In this publication, the optical head device guides the light flux emitted from the light source through the diffraction grating onto an optical recording medium in order to record and reproduce information into and from the optical recording medium. The diffraction grating is formed of an optically anisotropic polymer so as to exhibit the optically anisotropic characteristics.
In recent years, a super-combo drive that is capable of recording and reproducing for both CD and DVD with a single optical disk drive, has been put into practical use. The optical pickup used in a CD/DVD optical disk drive has a semiconductor laser with a wavelength of 790 nm used for CD and a semiconductor laser with a wavelength of 650 nm used for DVD, which are separated from each other. The light fluxes emitted from the semiconductor lasers (with the 650 nm wavelength the 790 nm wavelength) are synthesized on the same optical axis by means of a wavelength synthesizing prism. The synthesized light beam passes through a beam splitter, and is then collimated into parallel light by a collimating lens and made incident on an object lens. The light beam that has passed through the object lens is guided onto and reflected from the information recording surface of the optical recording medium. The reflected light (hereinafter, referred to as “signal light”) returns along the incident (forward) optical path.
In other words, the signal light is collimated into parallel light by the object lens, and focused onto the light-receiving surface of the photodetector via the collimating lens and the beam'splitter. The light is then converted into electric signals by the photodetector.
As a semiconductor laser for emitting light at two different wavelengths, a monolithic two-wavelength semiconductor laser is known. In this type of laser, a semiconductor laser of 790 nm wavelength and a semiconductor laser of 650 nm wavelength are monolithically formed in a single chip. Another type of two-wavelength semiconductor laser, in which multiple chips of semiconductor lasers with the respective wavelengths are arranged so that the distance between the light-emitting points becomes 100–300 μm, is also proposed. By using the above-described two-wavelength semiconductor laser, the number of components, the size, and the cost of the optical pickup can be decreased, as compared with a conventional optical pickup using two separated units of semiconductor lasers.
FIG. 2 schematically illustrates the structure of a two-wavelength semiconductor laser with laser chips of two different wavelength arranged very close to each other. The LDs (or semiconductor lasers) 102a and 102b with wavelengths for CD and DVD, respectively, are arranged so that the distance between the light emitting points is 100–300 μm. These laser chips 102a and 102b and PD (photodetector) 110 are packaged into a single package. Diffraction gratings 111a and 111b adapted for CD and DVD, respectively, are positioned in front of the closely arranged LDs 102a and 102b, in order to guide the signal lights to the PD 110 on the return path.
If these LDs 102a and 102b, the PD 110, and the diffraction gratings 111a and 111b are arranged in a single package, the beam diameter of the light for CD and that for DVD overlap each other on the return path, as illustrated in FIG. 2. For this reason, the diffraction grating 111a for CD has to have wavelength selectivity so as not to allow the DVD beam to be diffracted when the CD beam passes through the diffraction grating 111a. Similarly, the diffraction grating 111b for DVD has to have wavelength selectivity so as not to allow the CD beam to be diffracted when the DVD beam passes through the diffraction grating 111b. 
In addition, the diffraction gratings 111a and 111b have to be positioned in the vicinity of the LDs 102a and 102b. In order to guide the light to the PD 110 from this close position, the diffraction angle has to be set large at 15–20 degrees. In order to set the diffraction angle large, the grating pitch of the diffraction grating has to be narrowed. In view of the diffraction angle of 15–20 degrees, the pitch has to be set at about 2 microns.
With a diffraction grating having wavelength selectivity, when the wavelength selecting condition and the wavelength are determined, then the diffraction efficiency for the diffracted light with the selected wavelength is uniquely determined. This means that the diffraction efficiency cannot be set freely. To overcome this problem, JPA 2001-281432 discloses that for a wide pitch diffraction grating the maximum diffraction efficiency is obtained when the ratio of the width of the protrusion of the grating to the period is 0.5. Accordingly, JPA 2001-281432 proposes to set the ratio to a value other than 0.5 to decrease the diffraction efficiency, and to set diffraction efficiency arbitrarily in the region below the maximum diffraction efficiency.
However, the method disclosed in JPA 2001-281432 is only applicable to a wide pitch grating. In addition, the diffraction efficiency is only adjustable below the maximum diffraction efficiency. The diffraction efficiency in a narrow pitch region theoretically converges to about 30%, and it is difficult to achieve a diffraction efficiency exceeding this value.
As a technique for increasing the diffraction efficiency of a wide pitch diffraction grating, blazing is known. By blazing the shape of the grating, the efficiency ratio of the diffracted light is varied in the positive and negative directions. The diffraction efficiency can be increased by pulling the grating shape to one side. Although this method is effective for a wide pitch grating, it is not suitable for a narrow pitch grating because it is difficult to form a blaze in a narrow pitch grating due to the small pitch with respect to the depth of the groove. Consequently, it is difficult to provide a high efficiency diffraction grating.
For a recording DVD, a polarizing optical system is generally employed at present, and a polarizing diffraction grating with a return-path diffraction efficiency of about 32% is used. It is deemed that when the processing speed is further increased in the future, the quantity of light detected by the photodetector will become insufficient. In this case, it becomes difficult to realize a high-speed recording drive. If the diffraction efficiency of the polarizing diffraction grating can increase from the current level (32%), the quantity of light detected by the photodetector increases, and accordingly, a DVD recording drive capable of higher speed recording operation will be realized.
For an optical pickup used in an optical disk drive, such as Blu-Ray Disc, with the light source wavelength of 400 nm, a more highly efficient diffraction grating is required. The transmissivity of an optical element (such as a lens), which is 95% at a wavelength of 660 nm, decreases to 90% at a wavelength of 400 nm. In addition, since many optical elements for correcting various aberrations, such as spherical aberration, coma, and other aberrations, are inserted in the optical path, reduction of light quantity occurs every time light flux passes through an optical element. Furthermore, the photoelectric converting efficiency of the photodetector decreases due to decrease in the quantum efficiency caused by short wavelength, and therefore, the quantity of light detectable at the photodiode is further reduced. In view of the large reduction of the quantity of light detected at the photodetector, it is required for an optical pickup used in an optical disk drive using a wavelength of 400 nm to guarantee a diffraction efficiency of 60% or higher. It is difficult to produce such a high-efficiency optical pickup using a narrow pitch diffraction grating.
JPA 2002-288856 discloses an optical pickup using a hologram, as illustrated in FIG. 3. This optical pickup has a semiconductor laser chip 126 for emitting a prescribed light beam. The light beam emitted from the semiconductor laser chip 126 is split by the diffraction grating 125 into three beams, that is, two secondary beams for tracking and a primary beam for reading information signals. Thus, the diffraction grating 125 is used to produce extra tracking beams. These three beams pass through the hologram 124 as zero-order light, are then converted into parallel light by the collimating lens 123, and focused onto the disk (or the recording medium) 121 by the object lens 122. The light guided onto the disk 121 is modulated by the pit formed on the disc 121, and reflected from the disc 121. The reflected light passes through the object lens 122 and the collimating lens 123, and diffracted by the hologram 124. The diffracted light is guided onto the five-part photodiode 127 as first-order light.
In the optical pickup shown in FIG. 3, the hologram 124 is divided into multiple regions 124a and 124b. If the period of the grating varies between the regions, then the groove depth and the grating angle vary between the regions due to difference in etching rate during the hologram fabrication process, which further causes the diffraction efficiency to change between the regions on the hologram 124. Consequently, offset occurs in tracking signals. To overcome this problem, the invention disclosed in JPA 2002-288856 proposes that the multiple regions be arranged symmetrically using the boundary between the regions as the symmetric axis, while the grating periods of the respective regions are consistent with each other. This arrangement reduces variation in groove depth and grating angle, and therefore, variation in diffraction efficiency can be prevented. Especially, well-balanced tracking signals are produced with improved characteristics.
JPA 11-265515 discloses an optical pickup device using a diffraction element as shown in FIG. 4. In this optical pickup, the light emitted from the semiconductor laser 201 is diffracted by the diffraction element 202. The zero-order light component is guided through the polarizing beam splitter 203, the collimating lens 204, and the object lens 205 onto the recording medium 206. The return light reflected from the recording medium 206 passes through the object lens 205 and the collimating lens 204, and strikes the polarizing beam splitter 203. According to the polarization components, a portion of the light beam is reflected by the polarizing beam splitter 203 in the direction perpendicular to the return path, and guided to an optical system (not shown) for detecting information signals. The other portion of the light beam passes through the polarizing beam splitter 203, and is diffracted by the diffraction element 202. The first-order diffracted light is guided onto a light receiving element 207.
In the optical pickup shown in FIG. 4, the diffraction element 202 is divided into multiple regions. If the period (or the pitch) of the grating is large in one region, and small in the other region, as illustrated in the graph shown in FIG. 5A (that computes the distribution of the grating pitch on the diffraction element), then the groove is made deep in the wide pitch region, while the groove is made shallow in the narrow pitch region, during the etching process. The difference in groove depth causes the diffraction efficiency to vary, and offset occurs in tracking signal. To overcome this problem, JPA 11-265515 proposes to arrange the pitches of the grating so that the pitch in the farther region from the light receiving element becomes substantially the same as that in the closer region to the light receiving element, as shown in FIG. 5B, in order to reduce difference in diffraction efficiency. This arrangement allows well-balanced tracking signals to be produced with improved characteristics.
For a rewritable type optical pickup, employing an element that can make use of light at high efficiency is effective means for increasing the operating speed. For example, a polarization splitter having different diffraction efficiencies depending on the polarization directions can increase the light-using efficiency. JPA 2000-75130, which is assigned to the assignee of the present patent application, proposes a polarization splitter with a structure shown in FIG. 6. In this polarization splitter, a birefringent material layer 303 shaped into a periodic protrusion pattern is positioned over a transparent substrate 302. The birefringent material layer 303 is optically anisotropic and has two refractive indices with respect to different polarization planes of the incident light. The transparent substrate 302 and the birefringent material layer 303 are covered with anisotropic overcoat layer 304. The birefringent material layer 303 is made of a macromolecule material (for example, made of a drawn film of an organic polymer). Especially, it is easy for a drawn film of an organic polymer (hereinafter, referred to as an “organic drawn film”) to achieve a large area size, as compared with a crystalline material, such as LiNbO3, and therefore the cost can be reduced. The indices of refraction are near 1.6, and a highly transparent isotropic adhesive having a similar index of refraction can be easily obtained. Accordingly, the fabrication process for the polarization splitter is facilitated.